Meeting the commitments of the 2015 Paris climate agreement could save millions of lives and hundreds of billions of dollars by the middle of the century, according to the report.

Meeting the goals put forth in the Paris agreement would be expected to save more than 1 million lives a year from air pollution alone by 2050, it says. Drivers of climate change, principally fossil fuel combustion, contribute to about 7 million deaths worldwide from outdoor and indoor air pollution annually, according to the report.

The Paris Agreement is a pledge among representatives from countries around the world to take action against climate change, specifically by reducing their nations' carbon output and halting global warming below 2 degrees Celsius over pre-industrial levels by the end of the century. The United States pledged to withdraw from the landmark agreement last year.

Global leaders and officials are now gathering for two weeks of meetings at the 24th Conference of the Parties to the United Nations Framework Convention on Climate Change, or COP24, to create a rulebook that will turn the Paris climate agreement into a workable reality. They aim to establish rules, figure out financing and build ways to verify that nations are meeting their commitments.

UN Secretary-General Antonio Guterres called for action Monday in Poland, telling gathered delegates that "we are in deep trouble with climate change."

What needs to be done

The new WHO report, launched at the COP24, is based on contributions from more than 80 health professionals, academic experts and representatives of civil society and international agencies who have worked on climate change and public health for more than three decades.

"The most direct link between climate change and ill health is air pollution," the authors wrote.

"Burning fossil fuels for power, transport and industry is the main source of the carbon emissions that are driving climate change and a major contributor to health-damaging air pollution, which every year kills over seven million people due to exposure inside and outside their homes," according to the report.

The report provides recommendations for governments on how to tackle the issue of climate change.

Those recommendations include identifying and promoting actions to reduce both carbon emissions and air pollution; mobilizing mayors and other subnational leaders to promote climate goals; engaging the health community in addressing climate change; and systematically tracking progress in health from such climate change mitigation.

The report referenced an analysis showing that the value of the health gains from meeting the targets of the Paris climate agreement would be approximately twice the cost of the policies.

General climate change-related health impacts named in the report include in the areas of mental illness, undernutrition, injuries, respiratory disease, allergies, cardiovascular disease, infectious diseases, poisoning, waterborne diseases and heatstroke.

Only about 3% of health resources are invested in the prevention of climate change-related impacts, and only about 0.5% of multilateral climate funds have been dispersed specifically for health projects, according to the report.

'This isn't just a story about threats; it's a story about benefits'

The new WHO report is the fourth in the past two months to warn of the detrimental health impacts of climate change, said Dr. Mona Sarfaty, executive director of the Medical Society Consortium on Climate and Health and director of the program on climate and health at George Mason University's Center for Climate Change Communication. She was not involved in the report.

In October, the UN Intergovernmental Panel on Climate Change said in a report that the planet will reach the crucial threshold of 1.5 degrees Celsius (2.7 degrees Fahrenheit) above pre-industrial levels by as early as 2030, precipitating the risk of extreme drought, wildfires, floods and food shortages for hundreds of millions of people.

Then, in November, a separate report called The Lancet Countdown on Health and Climate Change showed how extreme heat from climate change already has been affecting productivity, food supply and disease transmission worldwide.

Also last month, the US government's National Climate Assessment warned that the economy could lose hundreds of billions of dollars by the end of the century due to climate change-related impacts.

The new WHO report comes with a message that "addressing climate change is an area of opportunity. It will improve our health, it will save money, and it will also stimulate economic development, because people who are healthier are able to be more productive," Sarfaty said. "The other reports share this message of possibility and potential for benefit."

As for the Paris Agreement, "there's no question that if we meet those goals, we'll save lives, and we will decrease the burden on the health delivery system, which will mean that people won't face as much poor health and won't end up in the hospital as frequently. Both -- that saving of lives and of health care services -- will save us money. So we save lives, we improve health, and we save money," she said.

"This isn't just a story about threats; it's a story about benefits we can gain if we go forward into a future powered by clean energy and highly efficient energy use," she said.

The drivers of climate change -- such as fossil fuel burning and large-scale livestock production -- are already posing a burden on public health, through air pollution and effects on respiratory and heart conditions, said Irva Hertz-Picciotto, a professor of public health sciences and director of the Environmental Health Sciences Center at UC Davis Health, who was not involved in the new report but has been studying the effects of recent wildfires in California on human health.

The air pollution from the California wildfires has big implications for the health of millions of people in the area. For instance, "after the 2017 Northern California fires were out -- Sonoma and Napa were two of the counties -- survivors who did not have a pre-existing respiratory condition were reporting respiratory symptoms still six months out," Hertz-Picciotto said.

"So that's some of what we're seeing," she said. "And that's just one tiny piece" of this larger discussion around climate change and health.

As mentioned in the new WHO report, "at the local level people can make really important changes, and that can help empower communities and in fact make meaningful changes at those local levels that will both reduce greenhouse gas emissions and be helpful in improving health and in terms of making cities more livable," she said. "One of the main -- and critical -- messages in this report is that you can't really separate climate changes from health -- both in the short-run and the long-run."